Process for inhibiting polymerization of an aromatic vinyl compound

ABSTRACT

A process for inhibiting polymerization of an aromatic vinyl compound during the stage of producing, purifying, storing or transporting the aromatic vinyl compound which not only inhibits an initial polymerization but also inhibits polymerization efficiently over a long period of time and which is excellent in the handleability. The process involves the step of adding a nitrogenous aromatic compound in combination with a sulfonic acid compound to the aromatic vinyl compound during the production, purification, storage of transportation thereof.

This is an application filed under 35 U.S.C. 371 of PCT/JP2007/055968, filed Mar. 23, 2007, which claims priority from JP 2006-082571, filed Mar. 24, 2006 and JP 2006-082572, filed Mar. 24, 2006.

TECHNICAL FIELD

The present invention relates to a process for inhibiting polymerization of an aromatic vinyl compound or of a process fluid containing it during the stage of producing, purifying, storing or transporting the aromatic vinyl compound.

BACKGROUND ART

Among aromatic vinyl compounds, styrene especially is an industrially very important compound as a raw material for preparing polystyrene, synthetic rubber, ABS resin, etc. and has been industrially manufactured in a large amount.

In general, an aromatic vinyl compound is extremely susceptible to polymerization so that it easily undergoes polymerization under an influence of the heat generated in its preparation or purification stage, thereby causing problems of lowering the yield of the monomeric aromatic vinyl compound and, further, of forming soil (fouling) in the facilities involved to induce trouble in operating the facilities. As the countermeasure taken hitherto, a process wherein a certain polymerization inhibitor is added to process fluid containing an aromatic vinyl compound has been proposed and practically carried out. For example, there have been proposed a process employing phenols, nitrosophenols and/or nitrophenols (e.g. patent literature 1), one employing piperidine-1-oxyls (e.g. patent literature 2), one employing nitrophenols together with piperidine-1-oxyls) (e.g. patent literature 3), one employing an alkylsulfonic acid in combination with a dialkylhydroxylamine (e.g. patent literature 4), one employing a phosphoric acid ester and an alkylsulfonic acid in combination (e.g. patent literature 5), one adding phenol-, amine- and/or nitroso-related polymerization inhibitors together with dodecylbenzenesulfonic acid or its salt in order to suppress an increase in viscosity during an olefin preparation process (e.g. patent literature 6), and one adding a quinone-, hydroquinone- or nitroso-related polymerization inhibitor, or an amine-related polymerization inhibitor such as phenylenediamine together with dodecylbenzenesulfonic acid or its salt for preventing the formation of fouling in stage treating a vinyl compound (e.g. patent literature 7). Among them, especially 2,4-dinitrophenol (DNP) and 2,6-dinitro-4-methylphenol, 2,4-dinitro-6-sec-butylphenol (DNBP), which are nitrophenol compounds, have been often used as the polymerization inhibitors for the aromatic vinyl compound. However, these chemical agents show a tendency to gradually decrease their inhibitory power toward the polymerization reaction and necessitate a strict care in handling them because they belong to toxic substances. Accordingly, there has been strongly desired a process by which the effect of these chemical agents on polymerization-inhibition may be maintained and the amount used of them may be reduced.

-   Patent literature 1: JP 63-316745 A -   Patent literature 2: JP 1-165534 A -   Patent literature 3: JP 6-166636 A -   Patent literature 4: U.S. Pat. No. 4,654,450 specification -   Patent literature 5: U.S. Pat. No. 4,425,223 specification -   Patent literature 6: JP 7-166152 A -   Patent literature 7: JP 8-34748 A

DISCLOSURE OF THE INVENTION Subject Matter to be Solved by the Invention

So, an object of the present invention is to provide a process for inhibiting polymerization of an aromatic vinyl compound during the stage of producing, purifying, storing or transporting of the aromatic vinyl compound which is capable of efficiently inhibiting polymerization not only in an initial stage but also over a long period of time and which is excellent in handleability.

Means for Solving the Subject Matter

As a result of having studied about characteristic properties of the polymerization reaction of an aromatic vinyl compound in detail, the present inventors have found that when a specific nitrogenous aromatic compound and a specific sulfonic acid compound were employed in combination, superior synergic polymerization-inhibiting effect may be obtained in comparison with the effect obtained when each of them was employed singly, and have completed the present invention.

That is, the invention is a process for inhibiting polymerization of an aromatic vinyl compound which comprises adding (A) at least one member selected from the nitrogenous aromatic compounds represented by the following general formulae (I), (II), (III) and (IV) in combination with (B) a sulfonic acid compound represented by the following general formula (V) during the stage producing, purifying, storing or transporting the aromatic vinyl compound

wherein R₁˜R₇ represent independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, a mono- or di-alkyl amino group whose alkyl group is a straight or branched chain alkyl having from 1 to 12 carbon atoms, a mono- or di-aryl amino group whose aryl group has from 6 to 12 carbon atoms, a formyl group, a carboxyl group, a carbamoyl group and a sulfonic acid residue, adjacent two substituent groups may take an alicyclic or an aromatic cyclic structure together, X represents oxygen or sulfur atom and Y represents oxygen or nitrogen atom

wherein R₈˜R₁₅ represent independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, a mono- or di-alkyl amino group whose alkyl group is a straight or branched chain alkyl having from 1 to 12 carbon atoms, a mono- or di-aryl amino group whose aryl group has from 6 to 12 carbon atoms, a formyl group, a carboxyl group, a carbamoyl group and a sulfonic acid residue, adjacent two substituent groups may take an alicyclic or an aromatic cyclic structure together, Y represents oxygen or nitrogen atom, X represents oxygen or sulfur atom and Z represents formic acid ion, acetic acid ion, propionic acid ion, sulfuric acid ion, chloride ion and nitric acid ion

wherein R₁₆˜R₂₅ represent independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, mono- or di-alkyl amino group whose alkyl group is a straight or branched chain alkyl having from 1 to 12 carbon atoms, a mono- or di-aryl amino group whose aryl group has from 6 to 12 carbon atoms, a formyl group, a carboxyl group, a carbamoyl group and a sulfonic acid residue, and adjacent two substituent groups may take an alicyclic or an aromatic cyclic structure together.

wherein R₂₆˜R₃₁ represent independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, an amino group substituted by at least one of straight or branched chain alkyl group having from 1 to 12 carbon atoms, an amino group substituted by at least one of aryl group having from 6 to 14 carbon atoms, and each of the two substituent groups of R₂₆ and R₂₇, of R₂₈ and R₂₉, of R₂₉ and R₃₀, and of R₃₀ and R₃₁ may bond together to form an alicyclic or an aromatic cyclic structure.

wherein R₃₂ represents hydroxyl group, a straight or branched chain alkyl group having 1-32 carbon atoms, an alkylphenyl group wherein the alkyl group is a straight or branched chain one having 1-32 carbon atoms or an alkylnaphthyl group wherein the alkyl group is a straight or branched chain alkyl having 1-32 carbon atoms.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the nitrogenous aromatic compound represented by the general formula (I) in the before-described (A) component is at least one member selected from methylene violet, resorufin and phenothiazine-3-oxide.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the nitrogenous aromatic compound represented by the general formula (II) in the before-described (A) component is at least one member selected from cresyl violet, thionine and methylene blue.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the nitrogenous aromatic compound represented by the general formula (III) in the before-described (A) component is indigo and/or indigo carmine.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the nitrogenous aromatic compound represented by the general formula (IV) in the before-described (A) component is at least one member selected from the group consisting of phenazine, 2-methylphenazine, 2-ethylphenazine, quinoxaline, 2-methylquinoxaline, 2-phenylquinoxaline, 3-amino-7-hydroxyl-2-methylphenazine and tetrabenzo[a, c, h, j]phenazine.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the sulfonic acid compound represented by the general formula (V) in the before-described (B) component is at least one member selected from toluenesulfonic acid, xylenesulfonic acid, cumenesulfonic acid, dodecylbenzene-sulfonic acid, pentadecylbenzenesulfonic acid and dinonyl-naphthalenesulfonic acid.

The invention is also the process for inhibiting polymerization of an aromatic vinyl compound, wherein the nitrogenous aromatic compound of the (A) component and the sulfonic acid compound of the (B) component are added in combination in the range of 5:95 to 95:5 by weight ratio.

Effect of the Invention

By the present invention, the polymerization of an aromatic vinyl compound or of a process fluid containing it may be efficiently inhibited and the effect may be kept for a long period of time thereby ensuring an advantage that the amount of fouling generated in the facilities involved may be minimized to attribute greatly to the safe operation of the facilities and to the reduction in the production cost.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The stages which become a target for inhibiting the polymerization in the present invention are: production stage of an aromatic vinyl compound, purification stage of the produced aromatic vinyl compound and storage stage of the purified aromatic vinyl compound, and they include attachment devices or equipment which are in contact with process liquid containing aromatic vinyl compound, and further circulation and recovery systems containing them. Specifically, a dehydrogenation reaction column or synthetic reaction column of an alkyl aromatic compound, discharge line or recovery and circulating line of the reaction product after the reaction in the production stage of an aromatic vinyl compound, piping for feeding an aromatic vinyl compound to the purification column, preheating line and cooling line and further circulating line in the purification stage, charge tank, storage tank, transfer tank and transport tank and further transfer lines thereof and so on in the storage stage are taken out.

As the aromatic vinyl compound, styrene and styrene derivatives having a polymerizable vinyl group such as an alkylstyrene whose alkyl has 1-10 carbon atoms, α-alkylstyrene where phenyl group or alkyl group each having 1-10 carbon atoms is located at the α-position, β-alkylstyrene where phenyl group or alkyl group each having 1-10 carbon atoms is located at the β-position and the like are taken. Specifically, styrene, methylstyrene (ortho form, meta form, para form and mixtures thereof), ethylstyrene (ortho form, meta form, para form and mixtures thereof), propylstyrene (ortho form, meta form, para form and mixtures thereof), butylstyrene (ortho form, meta form, para form and mixtures thereof), octylstyrene (ortho form, meta form, para form and mixtures thereof), nonylstyrene (ortho form, meta form, para form and mixtures thereof), decylstyrene (ortho form, meta form, para form and mixtures thereof), α-methylstyrene (2-phenylpropene), 1-phenyl-1-propene (β-methyl-styrene: cis form, trans form and mixtures thereof), 2-phenyl-2-butene (cis form, trans form and mixtures thereof), stilbene (cis form, trans form and mixtures thereof) and the like are taken.

The nitrogenous aromatic compounds represented by the general formulae (I)-(IV) as the (A) component which may be used for inhibiting the polymerization of these aromatic vinyl compounds have cyclic structure together with hetero atom. Among these compounds, compounds represented by the general formulae (I) and (II) are polycyclic imino compounds. Each of R₁˜R₇ in the general formula (I) and each of R₈˜R₁₅ in the general formula (II) represent independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, mono- or di-alkyl amino group whose alkyl group is a straight or branched chain alkyl having from 1 to 12 carbon atoms, mono- or di-aryl amino group whose aryl group has from 6 to 12 carbon atoms, a formyl group, a carboxyl group, a carbamoyl group and a sulfonic acid residue. Also, adjacent two substituent groups may take an alicyclic or an aromatic cyclic structure together. X represents oxygen or sulfur atom and Y represents oxygen or nitrogen atom. Z is not particularly restricted so long as it is an anion which forms a salt with a polycyclic imino compound. Usually, organic acid anions such as formic acid ion, acetic acid ion, propionic acid ion, inorganic acid ions such as chloride ion, sulfuric acid ion and nitric acid ion may be exemplified. Specific examples of a polycyclic imino compound include resorufin, methylene violet, phenothiazine-3-oxide, cresyl violet acetate, thionine and methylene blue.

Also, in the indigo compound represented by the general formula (III), each of R₁₆˜R₂₅ in the formula represents independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, mono- or di-alkyl amino group whose alkyl group is a straight or branched chain one having from 1 to 12 carbon atoms, a mono- or di-aryl amino group whose aryl group has from 6 to 12 carbon atoms, a formyl group, a carboxyl group, a carbamoyl group and a sulfonic acid residue, and adjacent two substituent groups may take an alicyclic or an aromatic cyclic structure together. Examples of an indigo compound include indigo, indigo carmine and the like. Although certain tautomer exists in these compounds, the indigo compounds include the tautomer.

Also, in the compound represented by the general formula (IV), each of R₂₆˜R₃₁ in the formula represents independently hydrogen, hydroxyl group, a straight or branched chain alkyl group having from 1 to 12 carbon atoms, a straight or branched chain alkenyl group having from 2 to 12 carbon atoms, an aryl group having from 6 to 14 carbon atoms, an amino group, an alkyl substituted amino group where at least one of straight or branched chain alkyl group having from 1 to 12 carbon atoms is located, an aryl-substituted amino group where at least one of aryl group having from 6 to 14 carbon atoms is located, and each of the two substituent groups of R₂₆ and R₂₇, of R₂₈ and R₂₉, of R₂₉ and R₃₀, and of R₃₀ and R₃₁ may bond together to form an alicyclic or an aromatic cyclic structure. As the specific compound, phenazine, 2-methylphenazine, 2-ethylphenazine, quinoxaline, 2-methyl-quinoxaline, 2-phenylquinoxaline, 3-amino-7-hydroxyl-2-methylphenazine, tetrabenzo[a, c, h, j]phenazine and the like are taken.

In the sulfonic acid compound represented by the general formula (V) as the (B) component which may be used in combination with these nitrogenous aromatic compounds represented by the general formulae (I)-(IV) as the (A) component, R₃₂ in the formula represents hydroxyl group, a straight or branched chain alkyl group having 1-32 carbon atoms, an alkyl substituted phenyl group or an alkyl substituted naphthyl group where at least one straight or branched chain alkyl group having 1-32 carbon atoms is located. One or more kinds of these sulfonic acid compounds may be selected. Examples of a sulfonic acid compound include sulfuric acid, toluenesulfonic acid, xylenesulfonic acid, cumenesulfonic acid, dodecylbenzene sulfonic acid, pentadecylbenzenesulfonic acid, dinonylnaphthalene-sulfonic acid and the like. Especially pentadecylbenzenesulfonic acid and dodecylbenzene sulfonic acid are preferred from the standpoint of view of their solubility toward aromatic vinyl compound, cost and so on.

The mixture ratio of the nitrogenous aromatic compound of the (A) component to the sulfonic acid compound of the (B) component is a matter to be adequately determined depending on the intended extent in the polymerization inhibition of the aromatic vinyl compound and hence cannot be determined as a rule. However, the nitrogenous aromatic compound of the (A) component and the sulfonic acid compound of the (B) component are added to the targeted aromatic vinyl compound in the range of generally 95:5 to 5:95, preferably 80:20 to 20:80, more preferably 70:30 to 30:70 by weight ratio.

The amount of the nitrogenous aromatic compound of the (A) component and of the sulfonic acid compound of the (B) component to be added to the targeted stage may be varied depending on the condition in the target stage, the degree required for the polymerization inhibition and so on and hence it cannot be determined as a rule. However, the nitrogenous aromatic compound of the (A) component is added in the range of generally 1-3000 ppm, preferably 10-2000 ppm, more preferably 100-1000 ppm to the targeted aromatic vinyl compound while the sulfonic acid compound of the (B) component is added in the range of generally 1-3000 ppm, preferably 10-2000 ppm, more preferably 100-1000 ppm thereto. The amount added of the respective components are values found as the proper range for exerting the polymerization-inhibiting effect toward the targeted aromatic vinyl compound. When they are smaller than this range, the effect is not sufficient. Also, when they are greater than the this range, the effect is sufficient but in some case it does not become great relative to the increase in the added amount and it is not preferable from the economical aspect.

The place in the target stage at which the nitrogenous aromatic compound of the (A) component and of the sulfonic acid compound of the (B) component are added in the present invention is not particularly restricted, they are added to the upstream process over the place at which an aromatic vinyl compound undergoes polymerization to cause the problem as fouling. For example, styrene is generally manufactured by a dehydrogenation reaction of ethyl benzene, and the formed styrene is continuously separated from the unreacted ethyl benbenzene by distillation, and therefore the nitrogenous aromatic compound and the sulfonic acid compound are supplied to the distillation columns after the dehydrogenation reaction of ethyl benzene has been conducted.

Although the means to be applied when the nitrogenous aromatic compound of the (A) component and the sulfonic acid compound of the (B) component are added to the targeted stage in the present invention is not particularly restricted, such a suitable means may be selected as they may be added to the specific place at one stroke or otherwise they may be separately added to some places on portions. In this case, although each of the before-described (A) and (B) components may be added separately, it is practically convenient that both the components are added in a proper mixture ratio and in a state dissolved in the same liquid as the process liquid, for example in case of being applied to styrene, in a state dissolved in ethyl benzene or crude styrene.

In the present invention, no limitation shall be imposed on that any other known polymerization inhibitor than the before described (A) and (B) components is used in combination in the range not impairing the effect of the present invention.

Working Example

The present invention is illustrated in more details by the working example but it is not restricted to the following working examples.

Compounds Used for Evaluation

(A) Component (Nitrogenous Aromatic Compound)

A-1: methylene violet (a product of Tokyo Kasei Kogyo Co., Ltd.)

A-2: resorufin (a product of Tokyo Kasei Kogyo Co., Ltd.)

A-3: cresyl violet (a product of Aldrich Chemical Company)

A-4: thionine (a product of Aldrich Chemical Company)

A-5: methylene blue (a product of Tokyo Kasei Kogyo Co., Ltd.)

A-6: indigo

A-7: indigo carmine

A-8: quinoxaline

A-9: 1-methyl quinoxaline

A-10: phenazine

A-11: 3-amino-7-hydroxyl-2-methylphenazine

A-12: tetrabenzo[a, c, h, j]phenazine

(B) Component (Sulfonic Acid Compound)

B-1: dodecylbenzenesulfonic acid

B-2: pentadecylbenzenesulfonic acid

B-3: xylenesulfonic acid

B-4: cumenesulfonic acid

B-5: p-toluenesulfonic acid

B-6: methansulfonic acid

B-7: sulfuric acid (98%)

(Others)

DNBP: 2,4 dinitro-6-sec-butyl phenol

H-TEMPO: 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl

Polymerization-Inhibiting Test-1

100 Grams of styrene monomer was placed in a four-necked separable flask equipped with a reflux condenser and the prescribed amount of the polymerization inhibitor shown in table 1 was added thereto (compounding ratios of each chemical agent was shown below), and highly purified nitrogen gas was injected therein for 30 minutes to remove the dissolved oxygen. Then, the resultant mixture was kept at 120° C. A part of the content therein was taken out as sampling at a given time to measure the amount (ppm) of polymer formed in the liquid. A 9 times volume of methanol based on the sampling liquid was added thereto to precipitate the formed polymer in a suspended state, which was filtrated out to weigh its weight amount and its formed amount (%) in the monomer was evaluated. Incidentally, before the beginning of this test, styrene monomer was washed with an alkali to remove the polymerization inhibitor (tert-butyl catechol) contained in the monomer, washed with water and dried. The results are summarized in table 1.

TABLE 1 (A) Nitrogeneous Aromatic (B) Sulfonic Acid Compound And Compound And Amount Amount Amount of Polymer formed (ppm) Example No. Added (ppm) Added (ppm) After 30 min. After 60 min. After 120 min. Working Example 1 A-1: 100 B-1: 100 2087 2987 3740 2 A-2: 100 B-1: 100 0 0 1331 3 A-3: 100 B-1: 100 87 341 11107 4 A-4: 100 B-1: 100 3471 7787 12361 5 A-5: 100 B-1: 100 0 1231 3771 6 A-2: 10 B-1: 190 1230 3210 6546 7 A-2: 40 B-1: 160 950 2010 5642 8 A-2: 60 B-1: 140 0 268 3210 9 A-2: 140 B-1: 60 0 521 2210 10 A-2: 160 B-1: 40 222 1125 2642 11 A-2: 190 B-1: 10 320 2846 3719 12 A-2: 100 B-2: 100 0 0 1667 13 A-2: 100 B-3: 100 0 45 2350 14 A-2: 100 B-4: 100 0 0 1024 15 A-2: 100 B-5: 100 0 59 1880 16 A-2: 100 B-6: 100 0 0 2129 17 A-2: 100 B-7: 50 66 112 6231 18 A-6: 100 B-1: 100 428 1159 3344 19 A-6: 100 B-2: 100 437 1182 3411 20 A-6: 100 B-3: 100 514 1391 4013 21 A-6: 100 B-4: 100 522 1414 4080 22 A-6: 100 B-5: 100 432 1171 3377 23 A-6: 100 B-6: 100 471 1275 3678 24 A-6: 100 B-7: 50 488 1722 4312 25 A-7: 100 B-1: 100 400 1250 3641 26 A-6: 10 B-1: 190 1620 3260 6621 27 A-6: 40 B-1: 160 578 1720 4121 28 A-6: 60 B-1: 140 523 1530 3821 29 A-6: 140 B-1: 60 628 1239 3662 30 A-6: 160 B-1: 40 1738 3487 7021 31 A-6: 190 B-1: 10 1923 3861 7822 Working Example 32 A-8: 100 B-1: 100 3028 5577 10120 33 A-9: 100 B-1: 100 3120 4979 8143 34 A10: 100 B-1: 100 151 163 248 35 A-11: 100 B-1: 100 2128 3549 7248 36 A-12: 100 B-1: 100 1720 2649 5439 37 A-10: 100 B-1: 200 66 97 101 38 A-10: 50 B-1: 50 1680 2939 4492 39 A-10: 10 B-1: 190 2890 4471 7022 40 A-10: 40 B-1: 160 110 135 212 41 A-10: 160 B-1: 40 98 120 144 42 A-10: 190 B-1: 10 3011 5670 11089 43 A-10: 100 B-2: 100 140 246 377 44 A-10: 100 B-3: 100 162 166 180 45 A-10: 100 B-4: 100 177 189 308 46 A-10: 100 B-5: 100 159 185 233 47 A-10: 100 B-6: 100 153 166 278 48 A-10: 100 B-7: 50 411 450 472 Comparative Example 1 DNBP: 250 None 1896 4929 16072 2 H-TEMPO: 250 None 0 0 42054 3 A-1: 200 None 38344 — — 4 A-2: 200 None 38791 — — 5 A-3: 200 None 34675 — — 6 A-4: 200 None 35681 — — 7 A-5: 200 None 32698 — — 8 A-6: 200 None 38344 — — 9 A-7: 200 None 39463 — — 10 A-10: 200 None 38385 — — 11 A-11: 200 None 39870 — — 12 A-12: 200 None 40010 — — 13 None B-1: 200 30700 — — 14 None B-2: 200 31314 — — 15 None B-3: 200 36840 — — 16 None B-4: 200 37454 — — 17 None B-5: 200 31007 — — 18 None B-6: 200 33770 — —

It can be understood from the results shown in table 1 that by combining the nitrogenous aromatic compound of the (A) component and the sulfonic acid compound of the (B) component the synergic polymerization-inhibiting effect may be exerted and the polymerization-inhibiting effect may be kept over a long period time while when each of them was used singly, almost no appreciable polymerization-inhibiting effect may be obtained. 

1. A process for inhibiting polymerization of an aromatic vinyl compound which comprises adding (A) at least one nitrogenous aromatic compound selected from the group consisting of phenazine, 3-amino-7-hydroxyl-2-methylphenazine, tetrabenzo [a,c,h,j] phenazine, quinoxaline and 2-methyl-quinoxaline, in combination with (B) a sulfonic acid compound selected from the group consisting of dodecylbenzene sulfonic acid, pentadecylbenzenesulfonic acid, xylenesulfonic acid, cumenesulfonic acid, p-toluenesulfonic acid and methanesulfonic acid, during the stage of producing, purifying, storing or transporting of the aromatic vinyl compound, wherein the nitrogenous aromatic compound is added in an amount of from 10-190 ppm and the sulfonic acid compound is added in an amount of from 10-200 ppm, based on the aromatic vinyl compound.
 2. The process as claimed in claim 1 wherein the nitrogenous aromatic compound of the (A) component and the sulfonic acid compound of the (B) component are added in combination in the range of 5:95 to 95:5 by weight ratio. 